I am reading American Psycho and trying to compare it with Edgar Allan Poe short stories in which I have to make a question for it Long story short both characters want something (the satisfaction of killing others) but when they get it (killing them) they don’t feel satisfied... I’ve never done this before but yah
You can regard such condition as insatiability (noun).
1. The condition of being insatiable - while insatiable is an adjective that denotes: (of an appetite or desire) impossible to satisfy. Note that the word insatiable/insatiability doesn't neccessarily denotes a desire of eating more and more, it can be used to denote other type of desires e.g. of power, of killing people, etc.
An example of insatiability is a lust of power in any corrupt leader, who never becomes satisfied with what position he has.
An example of insatiable is a desire for more and more money that never goes away.
An example of insatiable is a terrorist who keeps killing the people at bulk, but never becomes satisfied with how many people they have killed.
That cannot be overcome, subdued, appeased, or dispelled.
But it’s matched by an equally teeming, unquenchable emptiness. Los Angeles Times Jul 18, 2018
and your question:
After the killings, their criminal lust is still unquenched!